cell towers are typically 6 to 12 miles apart (less in cities) and a phone is usually within range of at least three of them.

This statement may be true in across most of the USA east of the Rockies. It most definitely is not true in the hinterlands of OR and WA, where the local populations are very small, mountains are very plentiful, and celluar phone companies have zero incentive to erect large numbers of cell towers.

Here in Oregon, I'd guess that density of cell phone coverage described in that quote applies to less than 15% of the state. CA has very similar issues in the Sierra Nevada and NoCal.